The present invention relates to a push button switch for a keyboard to be used as various input devices in a personal computer, a word processor or the like.
Conventionally, such a push button switch is known from U.S. Pat. No. 4,467,160, for example.
FIG. 7 shows such a conventional push button switch in vertical section. Referring to FIG. 7, the push button switch generally consists of a lower case 1, an upper case 2, a fixed contact member 3, a movable contact member 4, a stem 5 and a coil spring 6.
The lower case 1, the upper case 2 and the stem 5 are formed of insulating synthetic resin such as polyacetal.
The lower case 1 is of a generally box-like shape having an open top. A cylindrical guide projection 7 is formed to project upwardly from an inner bottom surface of the lower case 1 at a central portion thereof. The bottom portion of the lower case 1 between the guide projection 7 and a side wall of the lower case 1 is formed with slots 8 and 9 for inserting therethrough a fixed contact terminal 17 of the fixed contact member 3 and a movable contact terminal 22 of the movable contact member 4, respectively.
The upper case 2 is of a generally box-like shape having an open bottom, and is formed at its top surface with a central opening 10 for inserting the stem 5 therethrough. An engagement element (not shown) to be engaged with a stepped portion (not shown) of the lower case 1 depends from a side wall of the upper case 2. Thus, the engagement element is engaged with the stepped portion to assemble the lower case 1 with the upper case 2.
The stem 5 comprises a base portion 12 wider than the opening 10 of the upper case 2, an upward projection 12 projecting upwardly from the base portion 12 at a central portion thereof and adapted to be inserted through the opening 10, and a downward projection 13 projecting downwardly from the base portion 11 at the central portion thereof and adapted to be inserted into the guide projection 7 of the lower case 1.
The coil spring 6 has a lower end portion 14 mounted on the outer periphery of the guide projection 7 and has an upper end portion 15 abutting against a lower surface of the base portion 11 of the stem 5, thus normally biasing the stem 5 upwardly.
The fixed contact member 3 is formed from a relatively rigid metal plate having good conductivity, and is integrally formed with a contact portion 16 and a fixed contact terminal 17 depending from a lower end of the contact portion 16.
The movable contact member 4 is formed from a resilient sheet metal such as phosphor bronze, and is integrally formed with a movable contact wide element 18 bent at the midway thereof, a contact portion 19 formed at one lower end of the contact element 18, and a pair of substantially V-shaped stem abutting portions 21 formed on opposite sides of the contact portion 19 and adapted to be urged by a pair of inclined portions 20 of the stem 5. The movable contact terminal 22 depends from the other lower end of the contact element 18.
In operation, when the stem 5 is in an undepressed or rest position, the base portion 11 of the stem 5 is biased upwardly by the coil spring 6 to an upper limit position in the upper case 2, and the stem abutting portions 21 of the movable contact member 4 is in abutment against crests of the inclined portions 20 of the stem 5. Accordingly, the contact portion 19 of the movable contact member 4 is separate from the contact portion 16 of the fixed contact member 3. Under this condition, when the stem 5 is depressed against the biasing force of the coil spring 6, the stem abutting portions 21 of the movable contact member 4 are slidingly moved along the inclined portions 20 of the stem 5 toward the stem 5 (toward the fixed contact member 3). When the stem 5 is lowered near a lower limit position, the contact portion 19 of the movable contact member 4 is brought into contact with the contact portion 16 of the fixed contact member 3, thereby turning the switch on. On the other hand, when the depression of the stem 5 is released, the stem 5 is raised by the biasing force of the coil spring 6 to move the contact portion 19 of the movable contact member 4 away from the contact portion 16 of the fixed contact member 3, thereby turning the switch off.
The terminals 17 and 22 of the movable contact member 4 and the fixed contact member 3 are soldered to a land of a printed substrate (not shown), and an outer side wall of the case of the push button switch is mounted on a panel substrate (not shown).
In the conventional push button switch as mentioned above, upon soldering of the fixed contact terminal 17 and the movable contact terminal 22 to the printed substrate, flux is applied to the terminals 17 and 22 so as to improve the solderability. However, since the flux has a good wettability to a metal surface, the flux tends to be led along the terminals 17 and 22 and be deposited to the contact portions 16 and 19. Further, when the flux enters the case through the terminals 17 and 22, it tends to be gathered at corner portions 23 in the lower case 1 due to capillarity and surface tension and be raised on the inner wall surface of the lower case 1 along the corner portions 23, resulting in depositing to the contact portions 16 and 19 and causing a defective contact.
Furthermore, the lower flat surface of the lower case 1 is entirely mounted on the printed substrate. Therefore, when the stem 5 is depressed, and the lower end of the stem 5 abuts against the inner bottom surface of the lower case 1, a large impact sound is generated.